1. Field of the Invention
This invention relates to cascode current mirrors, in general, and to the microelectronic reproduction of a reference current for use in a binary-weighted current digital-to-analog converter, in particular.
2. Description of the Related Art
As is known, current sources are widely used in microelectronic circuitry as biasing elements and as load devices for various types of amplifier stages. As is also known, such use of current sources in biasing arrangements prove advantageous in the superior insensitivity of circuit performance to power supply variations and to changes in temperature which are oftentimes present. When used as a load element in transistor amplifier stages, furthermore, the high incremental resistance exhibited by the current source leads to high voltage gains at low power supply voltages. Because of these characteristics, a desirable application for a current source is in the binary-weighted current digital-to-analog converter. In such uses, a cascode current source employing MOS field-effect transistors is commonly employed, offering an accurate reproduction of the reference current.
One of the most important aspects of current-source performance with these MOS transistors, however, is the variation of current which results in the cascode mirror due to drain-source voltage changes at the output terminal. As will be appreciated by those skilled in the art, this can be characterized by the small signal output resistance of the current source. When the MOS transistors are used in the cascode current source mode, its small signal output resistance is typically set forth as: EQU R.sub.o =r.sub.o2 [1+(g.sub.m2 +g.sub.mb2)r.sub.o1 ]+r.sub.o1
Where r.sub.o2 represents the output resistance of one of the MOS transistors in the output pair, r.sub.o1 equals the output resistance of the other MOS transistor, g.sub.m2 is the transconductance of the first transistor, and g.sub.mb2 is the bulk transconductance of the first transistor. R.sub.o, in such formulation, represents the small signal output resistance of the circuit.
In actual circuit operation, on the other hand, the output voltage can vary (i.e., anywhere from ground to the supply voltage), with the resultant change that the reproduced current will vary as well. Thus, it would be beneficial if the output resistance of the cascode current mirror could somehow be increased so that any change in the output voltage would result only in a very small change in the output current.